gen_server

Generic Server Behaviour

A behaviour module for implementing the server of a client-server
relation. A generic server process (gen_server) implemented using
this module will have a standard set of interface functions and
include functionality for tracing and error reporting. It will
also fit into an OTP supervision tree. Refer to
OTP Design Principles for more information.

A gen_server assumes all specific parts to be located in a
callback module exporting a pre-defined set of functions.
The relationship between the behaviour functions and the callback
functions can be illustrated as follows:

If a callback function fails or returns a bad value,
the gen_server will terminate.

A gen_server handles system messages as documented in
sys(3). The sys module
can be used for debugging a gen_server.

Note that a gen_server does not trap exit signals automatically,
this must be explicitly initiated in the callback module.

Unless otherwise stated, all functions in this module fail if
the specified gen_server does not exist or if bad arguments are
given.

The gen_server process can go into hibernation
(see erlang(3)) if a callback
function specifies 'hibernate' instead of a timeout value. This
might be useful if the server is expected to be idle for a long
time. However this feature should be used with care as hibernation
implies at least two garbage collections (when hibernating and
shortly after waking up) and is not something you'd want to do
between each call to a busy server.

Creates a gen_server process as part of a supervision tree.
The function should be called, directly or indirectly, by
the supervisor. It will, among other things, ensure that
the gen_server is linked to the supervisor.

The gen_server process calls Module:init/1 to
initialize. To ensure a synchronized start-up procedure,
start_link/3,4 does not return until
Module:init/1 has returned.

If ServerName={local,Name} the gen_server is
registered locally as Name using register/2.
If ServerName={global,GlobalName} the gen_server is
registered globally as GlobalName using
global:register_name/2. If no name is provided,
the gen_server is not registered.

Module is the name of the callback module.

Args is an arbitrary term which is passed as
the argument to Module:init/1.

If the option {timeout,Time} is present,
the gen_server is allowed to spend Time milliseconds
initializing or it will be terminated and the start function
will return {error,timeout}.

If the option {debug,Dbgs} is present,
the corresponding sys function will be called for each
item in Dbgs. See
sys(3).

If the option {spawn_opt,SOpts} is present,
SOpts will be passed as option list to
the spawn_opt BIF which is used to spawn
the gen_server. See
erlang(3).

Note!

Using the spawn option monitor is currently not
allowed, but will cause the function to fail with reason
badarg.

If the gen_server is successfully created and initialized
the function returns {ok,Pid}, where Pid is
the pid of the gen_server. If there already exists a process
with the specified ServerName the function returns
{error,{already_started,Pid}}, where Pid is
the pid of that process.

If Module:init/1 fails with Reason,
the function returns {error,Reason}. If
Module:init/1 returns {stop,Reason} or
ignore, the process is terminated and the function
returns {error,Reason} or ignore, respectively.

call(ServerRef, Request) -> Reply

call(ServerRef, Request, Timeout) -> Reply

Makes a synchronous call to the gen_server ServerRef
by sending a request and waiting until a reply arrives or a
timeout occurs. The gen_server will call
Module:handle_call/3 to handle the request.

ServerRef can be:

the pid,

Name, if the gen_server is locally registered,

{Name,Node}, if the gen_server is locally
registered at another node, or

{global,GlobalName}, if the gen_server is
globally registered.

Request is an arbitrary term which is passed as one of
the arguments to Module:handle_call/3.

Timeout is an integer greater than zero which
specifies how many milliseconds to wait for a reply, or
the atom infinity to wait indefinitely. Default value
is 5000. If no reply is received within the specified time,
the function call fails. If the caller catches the failure
and continues running, and the server is just late with the reply,
it may arrive at any time later into the caller's message queue.
The caller must in this case be prepared for this
and discard any such garbage messages that are two element
tuples with a reference as the first element.

The return value Reply is defined in the return value
of Module:handle_call/3.

The call may fail for several reasons, including timeout and
the called gen_server dying before or during the call.

The ancient behaviour of sometimes consuming the server
exit message if the server died during the call while
linked to the client has been removed in OTP R12B/Erlang 5.6.

multi_call(Name, Request) -> Result

multi_call(Nodes, Name, Request) -> Result

multi_call(Nodes, Name, Request, Timeout) -> Result

Nodes = [Node]

Node = atom()

Name = atom()

Request = term()

Timeout = int()>=0 | infinity

Result = {Replies,BadNodes}

Replies = [{Node,Reply}]

Reply = term()

BadNodes = [Node]

Makes a synchronous call to all gen_servers locally
registered as Name at the specified nodes by first
sending a request to every node and then waiting for
the replies. The gen_servers will call
Module:handle_call/3 to handle the request.

The function returns a tuple {Replies,BadNodes} where
Replies is a list of {Node,Reply} and
BadNodes is a list of node that either did not exist,
or where the gen_server Name did not exist or did not
reply.

Nodes is a list of node names to which the request
should be sent. Default value is the list of all known nodes
[node()|nodes()].

Name is the locally registered name of each
gen_server.

Request is an arbitrary term which is passed as one of
the arguments to Module:handle_call/3.

Timeout is an integer greater than zero which
specifies how many milliseconds to wait for each reply, or
the atom infinity to wait indefinitely. Default value
is infinity. If no reply is received from a node within
the specified time, the node is added to BadNodes.

When a reply Reply is received from the gen_server at
a node Node, {Node,Reply} is added to
Replies. Reply is defined in the return value of
Module:handle_call/3.

Warning!

If one of the nodes is not capable of process monitors,
for example C or Java nodes, and the gen_server is not started
when the requests are sent, but starts within 2 seconds,
this function waits the whole Timeout,
which may be infinity.

This problem does not exist if all nodes are Erlang nodes.

To avoid that late answers (after the timeout) pollutes
the caller's message queue, a middleman process is used to
do the actual calls. Late answers will then be discarded
when they arrive to a terminated process.

cast(ServerRef, Request) -> ok

ServerRef = Name | {Name,Node} | {global,GlobalName} | pid()

Node = atom()

GlobalName = term()

Request = term()

Sends an asynchronous request to the gen_server
ServerRef and returns ok immediately, ignoring
if the destination node or gen_server does not exist.
The gen_server will call Module:handle_cast/2 to
handle the request.

Request is an arbitrary term which is passed as one
of the arguments to Module:handle_cast/2.

abcast(Name, Request) -> abcast

abcast(Nodes, Name, Request) -> abcast

Nodes = [Node]

Node = atom()

Name = atom()

Request = term()

Sends an asynchronous request to the gen_servers locally
registered as Name at the specified nodes. The function
returns immediately and ignores nodes that do not exist, or
where the gen_server Name does not exist.
The gen_servers will call Module:handle_cast/2 to
handle the request.

reply(Client, Reply) -> Result

This function can be used by a gen_server to explicitly send
a reply to a client that called call/2,3 or
multi_call/2,3,4, when the reply cannot be defined in
the return value of Module:handle_call/3.

Client must be the From argument provided to
the callback function. Reply is an arbitrary term,
which will be given back to the client as the return value of
call/2,3 or multi_call/2,3,4.

The return value Result is not further defined, and
should always be ignored.

enter_loop(Module, Options, State)

enter_loop(Module, Options, State, ServerName)

enter_loop(Module, Options, State, Timeout)

enter_loop(Module, Options, State, ServerName, Timeout)

Module = atom()

Options = [Option]

Option = {debug,Dbgs}

Dbgs = [Dbg]

Dbg = trace | log | statistics

| {log_to_file,FileName} | {install,{Func,FuncState}}

State = term()

ServerName = {local,Name} | {global,GlobalName}

Name = atom()

GlobalName = term()

Timeout = int() | infinity

Makes an existing process into a gen_server. Does not return,
instead the calling process will enter the gen_server receive
loop and become a gen_server process. The process
must have been started using one of the start
functions in proc_lib, see
proc_lib(3). The user is
responsible for any initialization of the process, including
registering a name for it.

This function is useful when a more complex initialization
procedure is needed than the gen_server behaviour provides.

Module, Options and ServerName have
the same meanings as when calling
gen_server:start[_link]/3,4.
However, if ServerName is specified, the process must
have been registered accordingly before this function
is called.

State and Timeout have the same meanings as in
the return value of
Module:init/1.
Also, the callback module Module does not need to
export an init/1 function.

Failure: If the calling process was not started by a
proc_lib start function, or if it is not registered
according to ServerName.

CALLBACK FUNCTIONS

The following functions
should be exported from a gen_server callback module.

If the initialization is successful, the function should
return {ok,State}, {ok,State,Timeout} or {ok,State,hibernate}, where
State is the internal state of the gen_server.

If an integer timeout value is provided, a timeout will occur
unless a request or a message is received within
Timeout milliseconds. A timeout is represented by
the atom timeout which should be handled by
the handle_info/2 callback function. The atom
infinity can be used to wait indefinitely, this is
the default value.

If hibernate is specified instead of a timeout value, the process will go
into hibernation when waiting for the next message to arrive (by calling
proc_lib:hibernate/3).

If something goes wrong during the initialization
the function should return {stop,Reason} where
Reason is any term, or ignore.

From is a tuple {Pid,Tag} where Pid is
the pid of the client and Tag is a unique tag.

State is the internal state of the gen_server.

If the function returns {reply,Reply,NewState},
{reply,Reply,NewState,Timeout} or
{reply,Reply,NewState,hibernate}, Reply will be
given back to From as the return value of
call/2,3 or included in the return value of
multi_call/2,3,4. The gen_server then continues
executing with the possibly updated internal state
NewState. See Module:init/1 for a description
of Timeout and hibernate.

If the functions returns {noreply,NewState},
{noreply,NewState,Timeout} or {noreply,NewState,hibernate},
the gen_server will
continue executing with NewState. Any reply to
From must be given explicitly using
gen_server:reply/2.

If the function returns {stop,Reason,Reply,NewState},
Reply will be given back to From. If
the function returns {stop,Reason,NewState}, any reply
to From must be given explicitly using
gen_server:reply/2. The gen_server will then call
Module:terminate(Reason,NewState) and terminate.

See Module:handle_call/3 for a description of
the arguments and possible return values.

Module:handle_info(Info, State) -> Result

Info = timeout | term()

State = term()

Result = {noreply,NewState} | {noreply,NewState,Timeout}

| {noreply,NewState,hibernate}

| {stop,Reason,NewState}

NewState = term()

Timeout = int()>=0 | infinity

Reason = normal | term()

This function is called by a gen_server when a timeout
occurs or when it receives any other message than a
synchronous or asynchronous request (or a system message).

Info is either the atom timeout, if a timeout
has occurred, or the received message.

See Module:handle_call/3 for a description of
the other arguments and possible return values.

Module:terminate(Reason, State)

Reason = normal | shutdown | {shutdown,term()} | term()

State = term()

This function is called by a gen_server when it is about to
terminate. It should be the opposite of Module:init/1
and do any necessary cleaning up. When it returns,
the gen_server terminates with Reason. The return
value is ignored.

Reason is a term denoting the stop reason and
State is the internal state of the gen_server.

Reason depends on why the gen_server is terminating.
If it is because another callback function has returned a
stop tuple {stop,..}, Reason will have
the value specified in that tuple. If it is due to a failure,
Reason is the error reason.

If the gen_server is part of a supervision tree and is
ordered by its supervisor to terminate, this function will be
called with Reason=shutdown if the following
conditions apply:

the gen_server has been set to trap exit signals, and

the shutdown strategy as defined in the supervisor's
child specification is an integer timeout value, not
brutal_kill.

Even if the gen_server is not part of a supervision tree,
this function will be called if it receives an 'EXIT'
message from its parent. Reason will be the same as in
the 'EXIT' message.

Otherwise, the gen_server will be immediately terminated.

Note that for any other reason than normal,
shutdown, or {shutdown,Term} the gen_server is
assumed to terminate due to an error and
an error report is issued using
error_logger:format/2.

Module:code_change(OldVsn, State, Extra) -> {ok, NewState}

OldVsn = Vsn | {down, Vsn}

Vsn = term()

State = NewState = term()

Extra = term()

This function is called by a gen_server when it should
update its internal state during a release upgrade/downgrade,
i.e. when the instruction {update,Module,Change,...}
where Change={advanced,Extra} is given in
the appup file. See
OTP Design Principles
for more information.

In the case of an upgrade, OldVsn is Vsn, and
in the case of a downgrade, OldVsn is
{down,Vsn}. Vsn is defined by the vsn
attribute(s) of the old version of the callback module
Module. If no such attribute is defined, the version
is the checksum of the BEAM file.

State is the internal state of the gen_server.

Extra is passed as-is from the {advanced,Extra}
part of the update instruction.

The function should return the updated internal state.

Module:format_status(Opt, [PDict, State]) -> Status

Opt = normal | terminate

PDict = [{Key, Value}]

State = term()

Status = term()

Note!

This callback is optional, so callback modules need not
export it. The gen_server module provides a default
implementation of this function that returns the callback
module state.

This function is called by a gen_server process when:

One
of sys:get_status/1,2
is invoked to get the gen_server status. Opt is set
to the atom normal for this case.

The gen_server terminates abnormally and logs an
error. Opt is set to the atom terminate for this
case.

This function is useful for customising the form and
appearance of the gen_server status for these cases. A
callback module wishing to customise
the sys:get_status/1,2 return value as well as how
its status appears in termination error logs exports an
instance of format_status/2 that returns a term
describing the current status of the gen_server.

PDict is the current value of the gen_server's
process dictionary.

State is the internal state of the gen_server.

The function should return Status, a term that
customises the details of the current state and status of
the gen_server. There are no restrictions on the
form Status can take, but for
the sys:get_status/1,2 case (when Opt
is normal), the recommended form for
the Status value is [{data, [{"State",
Term}]}] where Term provides relevant details of
the gen_server state. Following this recommendation isn't
required, but doing so will make the callback module status
consistent with the rest of the sys:get_status/1,2
return value.

One use for this function is to return compact alternative
state representations to avoid having large state terms
printed in logfiles.